1. Field of the Invention
The present invention relates to a power source for producing a discharge between a wire electrode and a workpiece in wire-cut electric discharge machines.
2. Description of the Prior Art
An example of a known power source of this kind is, shown in FIG. 1. In FIG. 1, reference character WR indicates a wire electrode; WK designates a workpiece; Vs identifies a DC power supply; C denotes a capacitor; R represents a charging resistor; and Q shows a switching element such as a transistor or the like. By repeating the ON-OFF operation of the switching element Q, the capacitor C is charged by the DC power supply Vs via the charging resistor R. The charged voltage is applied across the wire electrode WR and the workpiece WK and a discharge current from the capacitor C flows in a discharge gap defined between the wire electrode WR and the workpiece WK, performing electric discharge machining of the latter.
For a stable discharge in electric discharge machining, a certain discharge suspension period is required for each discharge and, at the same time, care should also be taken while applying voltage during the period immediately following each discharge. The insulation recovery characteristic of the discharge gap after a discharge is schematically indicated by the solid line A in FIG. 2. When a voltage higher than the curve A (which voltage will hereinafter be referred to as the redischarge voltage) is applied across the discharge gap, it is likely that a discharge will be repeated at the same position or that an arc discharge will result.
With the view of settling this problem, in the prior art circuit of FIG. 1 the charging time constant of the capacitor C is selected to be large so that its charging characteristic curve may exceed the insulation recovery characteristic curve A, that is, the redischarge voltage, as indicated by the broken line B in FIG. 2. Accordingly, the frequency of discharge per unit time is limited and the cutting speed cannot be raised. Furthermore, the conventional circuit is a dependent discharge circuit, which may sometimes be discharged before the capacitor C is sufficiently charged and consequently the discharge energy may vary for each discharge, resulting in unsatisfactory surface roughness of the workpiece.